Devices which convert analog signals to digital signals are commonly referred to as A/D converters and are of great importance in modern technology. For example, in many applications, a signal, which may represent voltage, current, pressure, etc., is generated in analog form but further processing of the signal is desired. The additional processing is frequently more expeditiously performed if the analog signal is converted to a digital form so that the entire processing may be done digitally.
The A/D conversion is performed as the A/D converter samples the analog signal and then quantizes it to obtain a digital representation of the original analog signal. The subject of analog-to-digital converters is discussed generally by Taub and Schilling in Digital Integrated Electronics, pp. 486-543, McGraw-Hill, 1977. Although much of their discussion is applicable to A/D converters generally, the specific embodiments of A/D converters discussed by these authors are converters that perform the analog-to-digital conversion electronically. It will be readily appreciated by those skilled in the art that the accuracy of the analog-to-digital conversion depends upon the frequency at which the analog signal is sampled and quantized. This is true because the sampling frequency sets a limit to the highest frequency component of the analog signal which may be accurately determined. As a result, there is an interest in high speed A/D converters and, in particular, A/D converters that might be faster than electronic A/D converters.
One such A/D converter is the electrooptic converter, such as that discussed in Electronics Letters, 10, pp. 508-509, Nov. 28, 1974 which used a LiNb0.sub.3 diffraction modulator. Analog voltages are applied to the modulator and an array of photodiodes so that individual photodetectors detect particular orders of the diffraction pattern. A digital code is then generated by applying the voltages from the photodiodes to threshold detectors. Another electrooptic converter is discussed in Proceedings of the IEEE, pp. 1524-1525, Oct. 1975. The converter described was a three-bit electrooptic converter comprising a plurality of electrooptic waveguide modulators having electrodes of different lengths. Polarized cw light is incident on each waveguide and a signal voltage is applied to each waveguide. The phase of the TE mode is retraded with respect to the TM mode by an amount that depends on the voltage and the electrode length. The intensities of the two orthogonally polarized components of the light from each waveguide are detected separately, and the relative intensities of these components determine whether the value of the bit is 0 or 1 , i.e., each waveguide determines one bit. Another electrooptic analog-ditital converter is described in IEEE Journal of Ouantum Electronics, Vol. QE-18, pp. 1411-1413, Oct. 1982. This converter used a AlGaAs diode laser as the light source for the short optical pulses used for sampling and a LiNb0.sub.3 Ti indiffused interferometric waveguide array for analog-to-ditital conversion.